These sorts of "theme" movies often take the easy way out and rely on the genre alone to carry them to success. There's no doubt that many people will go to Letters to God simply because it's overtly Christian and buying a movie ticket is an easy way to send the message to Hollywood that we're willing to pay for wholesome messages. In the same way, if The Hobbit is ever made into a movie we can rest assured it will set box office records on the merit of its name alone; not necessarily the quality of the movie.
So, for the purpose of sending a message that we want more Christian-friendly films it may well be worth going to Letters to God. However, two questions remain: is the movie worth seeing apart from its theme and will the movie show Christianity in a believable and honest light?
My wife and I went to Letters to God last night and overall I was very pleased with the production and content of the movie. I have seen other overtly Christian movies like Fireproof and To Save a Life and found that the audio and video quality of Letters to God stands well above the other two (especially Fireproof. When I saw this in the theater the lips and voices didn't sync until halfway through the film).
That said; audio still was far under the standards of a major motion picture. There were a handful of scenes (mostly outdoors and with more action) where voices were either muffled or soft compared to the rest of the film. For the most part this wasn't an issue but it was enough that I noticed it.
Acting was strong, especially for this genre. In a movie where "God" is referenced literally hundreds of times it's very important to have actors who can convincingly pull it off and Letters to God did a good job considering the script they had to work with. Considering that God apparently had to be part of nearly every scene, these actors convincingly portrayed him as part of their lives.
Here we see the great challenge of this genre; how to display a clear Christian message in a natural way to which a broad audience can relate. Movies of this type tend to speak so candidly about God, as if his presence is assumed to be completely and perfectly integrated into everyday life, that referring to him is second nature. While this approach may be easy to relate to for many Christians; I fear it is alienating to non-Christian moviegoers.
In this area Letters to God is less forceful than, say, Fireproof and has some brilliant moments showing the struggle of trusting and finding God in the midst of suffering. Yet, it is still less a movie about a boy with cancer who is also Christian and more a movie about a Christian boy who also has cancer. The difference is subtle but detectable. If a moviegoer doesn't identify strongly with Christianity in the first place it will be very difficult to identify with any of the characters; which may put the entire message in jeopardy.
In the end, then, I feel Letters to God is a movie worth seeing on its own merit for the quality of production, acting, and the compelling story. However, its overt Christian nature fails to find middle ground on which Christian and non-Christian alike can relate and be touched by the characters and story.